The present invention relates to a method and apparatus for manufacturing polymer fiber shells via electrospinning.
Polymer fiber shells such as tubular shaped products, are used in the medical industry for various utilities including esophageal grafts, vascular grafts, stent coats and like.
Numerous methods for manufacturing polymer fiber shells suitable for medical applications are known in the art, including, for example, various injection molding methods, mandrel assisted extrusion or formation and various weaving techniques.
Production of polymer fiber shells suitable for use as vascular grafts is particularly difficult, since such grafts must withstand high and pulsatile blood pressures while, at the same time, be elastic and biocompatible.
Vascular grafts known in the art typically have a microporous structure that in general allows tissue growth and cell endothelization, thus contributing to long term engraftment and patency of the graft.
In vascular grafts, tissue ingrowth and cell endothelization is typically enhanced with increased in grafts exhibiting increased porosity. However, increasing the porosity of vascular grafts leads to a considerable reduction of the mechanical and tensile strength of the graft, and as a consequence to a reduction in the functionality thereof.
Electrospinning has been used for generating various products for medical applications, e.g., wound dressings, prosthetic devices, and vascular grafts as well as for industrial use, e.g., electrolytic cell diaphragms, battery separators, and fuel cell components. It has already been proposed to produce by electrospinning products having the appearance of shells. For example, U.S. Pat. No. 4,323,525 discloses a method of preparing a tubular product by electrostatically spinning a fiber forming material and collecting the resulting spun fibers on a rotating mandrel. U.S. Pat. No. 4,552,707 discloses a varying rotation rate mandrel which controls the “anisotropy extent” of fiber orientation of the final product. Additional examples of tubular shaped products and a like are disclosed, e.g., in U.S. Pat. Nos. 4,043,331, 4,127,706, 4,143,196, 4,223,101, 4,230,650 and 4,345,414.
The process of electrospinning creates a fine stream or jet of liquid that upon proper evaporation yields a non-woven fiber structure. The fine stream of liquid is produced by pulling a small amount of a liquefied polymer (either polymer dissolved in solvent (polymer solution) or melted polymer) through space using electrical forces. The produced fibers are then collected on a suitably located precipitation device, such as a mandrel to form tubular structures. In the case of a melted polymer which is normally solid at room temperature, the hardening procedure may be mere cooling, however other procedures such as chemical hardening or evaporation of solvent may also be employed.
In electrospinning, an electric field with high filed lines density (i.e., having large magnitude per unit volume) may results in a corona discharge near the precipitation device, and consequently prevent fibers from being collected by the precipitation device. The filed lines density of an electric field is determined inter alia by the geometry of the precipitation device; in particular, sharp edges on the precipitation device increase the effect of corona discharge.
In addition, due to the effect of electric dipole rotation along the electric field maximal strength vector in the vicinity of the mandrel, products with at least a section with a small radius of curvature are coated coaxially by the fibers. Such structural fiber formation considerably reduces the radial tensile strength of a spun product, which, in the case of vascular grafts, is necessary for withstanding pressures generated by blood flow.
Various electrospinning based manufacturing methods for generating vascular grafts are known in the prior art, see, for example, U.S. Pat. Nos. 4,044,404, 4,323,525, 4,738,740, 4,743,252, and 5,575,818. However, such methods suffer from the above inherent limitations which limit the use thereof when generating intricate profile fiber shells.
Hence, although electrospinning can be efficiently used for generating large diameter shells, the nature of the electrospinning process prevents efficient generation of products having an intricate profile and/or small diameter, such as vascular grafts. In particular, since porosity and radial strength are conflicting, prior art electrospinning methods cannot be effectively used for manufacturing vascular grafts having both characteristics.
There is thus a widely recognized need for, and it would be highly advantageous to have, a method and apparatus for manufacturing polymer fiber shells via electrospinning devoid of the above limitations.